Chapter 11
Introduction to Genetics
11-1
The Work of Gregor Mendel
Gregor Mendel’s Peas
• Gregor Mendel was an Austrian monk who spent
several years studying science and math. He took
charge of the monastery garden and had several
different stocks of pea plants. These peas were
true-breeding, which means that if they were
allowed to self-pollinate, they would produce
offspring identical to themselves. One stock of
seeds would produce only tall plants, another
only short plants. One stock produced only green
seeds, another only yellow seeds.
Pea plants can also cross-pollinate. This is when
male sex cells in pollen from the flower on one
plant fertilize the egg cells of a flower on
another plant. The seeds produced from crosspollination have two plants as parents.
Mendel took two different pea plants. To
prevent them from self-pollinating, he cut away
the male parts of a flower. Then he dusted that
flower with pollen from a second flower.
Genes and Dominance
• Mendel studies seven different pea plant
traits.
A trait is a specific characteristic, such as seed
color or plant height, that varies from one
individual to another.
Mendel crossed plants with each of the seven
contrasting characters and studied their
offspring. Mendel called each original pair of
plants the P (parental) generation. He called the
offspring the F1, or “first filial” generation. The
offspring of crosses between parents with
different traits are called hybrids.
To Mendel’s surprise, all of the offspring had the
character of only one of the parents (not
blended). The character of the other parent
seemed to have disappeared.
Mendel drew two conclusions:
1- Biological inheritance is determined by factos
that are passed from one generation to the
next
• Genes – the chemical factors that determine
traits
• Alleles- the different forms of a gene
2- The principle of dominance – some alleles are
dominant and others are recessive
An organism with a dominant allele for a
particular form of a trait will always have that
form. An organism with a recessive allele for a
particular form of a trait will have that form only
when the dominant allele for the trait is not
present.
In Mendel’s experiments, the allele for tall
plants was dominant and the allele for short
plants was recessive. The allele for yellow seeds
was dominant, while the allele for green seeds
was recessive.
Segregation
• Mendel wanted to know if the recessive
alleles disappeared or if they were still present
in the F1 plants. To find out, he allowed all
seven kinds of F1 hybrid plants to produce an
F2 generation by self-pollination. He found
that the traits controlled by the recessive
alleles reappeared. About one fourth of the
F2 plants showed the trait controlled by the
recessive allele.
Explaining the F1 Cross
• Mendel wanted to understand how this
segregation (separation) occurs. He believed
that the alleles for tallness and shortness in
the F1 plants were segregated from each
other during the formation of the gametes
(sex cells).
When each F1 plant flowers, the two alleles are
segregated from each other so that each gamete
carries only a single copy of each gene.
Therefore, each F1 plant produces two types of
gametes – those with the allele for tallness and
those with the allele for shortness.
11-2 PROBABILITY AND PUNNETT
SQUARES
Genetics and Probability
• Probability – the likelihood that a particular
event will occur
• The principles of probability can be used to
predict the outcomes of genetic crosses
• For example, when you flip a coin, there are
two possible outcomes: land on heads or tails
The probability that a single coin flip will come
up heads id ½. If you flip a coin 3 times in a row,
what is the probability that it will land on
heads? Each flip is an independent event, so the
probability of each coin’s landing on heads is ½.
Therefore, the probability of flipping three
heads in a row is ½ x ½ x ½
There is a 1/8 chance of flipping heads three
times in a row. The fact that each flip is an
independent event means that past outcomes
do not affect future ones.
Punnett Squares
• The gene combinations that might result from
a genetic cross can be determined by drawing
a Punnett square. The letters in the Punnett
square represent alleles: capital letters for
dominant alleles and lowercase letters for
recessive alleles.
• Organisms that have two identical alleles for a
particular trait (TT or tt) are said to be
homozygous.
• Organisms that have two different alleles for
the same trait are heterozygous.
Homozygous organisms are true-breeding for a
particular trait. Heterozygous organisms are
hybrid for a particular trait.
• Phenotype – physical characteristics
• Genotype – genetic makeup
All of the tall plants have the same phenotype
but have different genotypes. The genotype of
one third of the tall plants is TT. The genotype
of two thirds of the tall plants is Tt.
11-3 EXPLORING MENDELIAN
GENETICS
Independent assortment
• Mendel performed an experiment to follow
two different genes as they passed from one
generation to the next. Mendel’s experiment
is known as a two-factor cross.
First, Mendel crossed true-breeding plants that
produced only round yellow peas
(genotype RRYY) with plants that produced
wrinkled green peas (genotype rryy). All of the
F1 offspring produced round yellow peas. This
shows that the alleles for yellow and round peas
are dominant over the alleles for green and
wrinkled peas.
ry
RY RrYy
RY RrYy
RY RrYy
RY RrYy
ry ry ry
RrYy RrYy RrYy
RrYy RrYy RrYy
RrYy RrYy RrYy
RrYy RrYy RrYy
This cross didn’t indicate whether genes
segregate independently, but it provides the
hybrid plants needed for the next cross – the
cross of F1 plants to produce the F2 generation.
Mendel knew that F1 plants were all
heterozygous for both the seed shape and seed
color genes. He wanted to know how the alleles
would segregate when the F1 plants were
crossed to each other to produce an F2
generation. He found that the alleles for seed
shape segregated independently of those for
seed color. This is called independent
assortment.
Independent assortment – genes for different
traits can segregate independently during the
formation of gametes.
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
RY
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
• A Summary of Mendel’s Principles
• The inheritance of biological characteristics is
determined by genes. Genes are passed from
parents to their offspring
• Some forms of genes may be dominant and
others may be recessive
• In most organisms, each adult has two copies
of each gene – one from each parent. These
genes are segregated from each other when
gametes are formed
• The alleles for different genes usually
segregate independently of one another
Beyond Dominant and Recessive Alleles
• Some alleles are neither dominant nor
recessive, and many traits are controlled by
multiple alleles or multiple genes
Incomplete Dominance
• A cross between two four o’clock plants, one
red and the other white, results in pink
colored flowers. Neither allele is dominant.
• Incomplete dominance – when one allele is
not completely dominant over another
Codominance
• When both alleles contribute to the
phenotype of the organism
• For example, in cattle the allele for red hair is
codominant with the allele for white hair.
Cattle with both alleles are pinkish brown
because their coats are a mixture of both red
and white hairs.
Multiple Alleles
• Genes that have more than two alleles are
said to have multiple alleles
• Ex – human genes for blood type
Polygenic Traits
• Traits controlled by two or more genes are
polygenic traits.
• Ex – at least 3 genes are involved in making
the reddish-brown pigment in the eyes of fruit
flies
• Ex – the wide range of human skin color
11-4 MEIOSIS
Mendel’s principles of genetics requires that
each organism must inherit a single copy of
every gene from both its “parents” and that
when an organism produces its own gametes,
those two sets of genes must be separated from
each other so that each gamete contains just
one set of genes.
Chromosome Number
• Diploid – a cell containing both sets of
homologous chromosomes
• Haploid – cells containing one set of
chromosomes and a single set of genes
Phases of Meiosis
• Meiosis – a process of reduction division in
which the number of chromosomes per cell is
cut in half through the separation of
homologous chromosomes in a diploid cell
Meiosis involves two distinct stages: meiosis I
(the first meiotic division) and meiosis II (the
second meiotic division)
Meiosis I
Interphase:
-the cell replicates its chromosomes
Prophase 1:
-Each chromosome pairs with its
corresponding homologous chromosome
to form a tetrad
Metaphase 1:
-the centromere of each chromosome
becomes attached to a spindle fiber
-the spindle fibers pull the tetrads to the
equator of the spindle
-homologous chromosomes are lined up
side by side as tetrads
Anaphase 1:
-homologous chromosomes separate and
move to opposite ends of the cell
-centromeres do not split
-this ensures that each new cell will
receive only one chromosome from each
homologous pair
Telophase 1:
-the spindle breaks down and the
chromosomes uncoil
-the cytoplasm divides into two new cells
-each cell has half the genetic information
of the original cell because it has only one
homologous chromosome from each pair
Meiosis II
Prophase II
-Meiosis I results in two haploid daughter
cells
- Each has half the number of
chromosomes as the original cell
Metaphase II-
-the chromosomes are pulled to the center
of the cell and line up randomly at the
center
Anaphase II-
-the centromere of each chromosome
splits
-the sister chromatids separate and move
to opposite poles
Telophase II-
-nuclei re-form
-the spindles break down
-the cytoplasm divides
By the end of meiosis II, the diploid cell that
entered meiosis has become 4 haploid daughter
cells
Gamete Formation
• In male animals, the haploid gametes
produced by meiosis are called sperm.
• In female animals, generally only one of the
cells produced by meiosis is involved in
reproduction. This female gamete is called an
egg in animals and an egg cell in some plants.
Comparing Mitosis and Meiosis
Mitosis and meiosis are very different.
Mitosis results in the production of two
genetically identical diploid cells, whereas
meiosis produces four genetically different
haploid cells.
• 11-5 LINKAGE AND GENE MAPS
Gene Linkage
Genes located on different chromosomes assort
Independently, but genes located on the same
chromosomes are inherited together
Each chromosome is actually a group of linked
genes
Mendel’s principle is true, but it is the
chromosomes that assort independently, not
individual genes
Gene Maps
Just because genes are on the same
chromosome does not mean that they are linked
forever. Crossing-over during meiosis
sometimes separates genes that have been on
the same chromosome onto homologous
chromosomes. Crossover events occasionally
separate and exchange linked genes and
produce new combinations of alleles. This
helps to generate genetic diversity.
In 1911, a Columbia University student named
Alfred Sturtevant hypothesized that how fast or
slow at which crossing over separated linked
genes could be important. He believed that the
farther apart the genes were, the more likely
that they were to be separated in a cross over
during meiosis
The rate at which linked genes are separated
and recombined could be used to produce a
“map” of distances between genes
Gene map – shows the relative location of each
gene on a chromosome